Arrow season 4: Producer tweets filming update on new crossover with The Flash - leading to Legends Of Tomorrow

The Flash (left) and Arrow are part of The CW's DC superhero universeThe CW

New crossover episodes with Arrow and The Flash are being filmed this week. These episodes will reportedly lead into the upcoming spinoff series - Legends Of Tomorrow. Arrow showrunner Marc Guggenheim recently teased a production update for the episode.

The said episode is titled Legends Of Yesterday and is the eighth instalment of the upcoming season 4. It "presumably focuses on the backstory of the ancient, immortal Vandal Savage, who will be the central antagonist of both the crossover and the Legends of Tomorrow series," speculates ComicBook.

Meanwhile, on the subject of crossovers, fans are also looking forward to Constantine appearing on Arrow in the forthcoming season and the official synopsis for episode 3 (below) seems to set the stage for his role in the following instalment. The description reveals that Laurel (Katie Cassidy) talks Thea (Willa Holland) into returning to Nanda Parbat to ask her father (John Barrowman) to put Sara (guest star Caity Lotz) into the Lazarus Pit. However, Laurel is surprised when Nyssa (guest star Katrina Law) refuses to do it. We guess that's where Constantine comes in.

Earlier reports have hinted that his one-episode track on the show will involve him bringing Sara back to life via the Lazarus Pit. Season 4 of Arrow is set to premiere on 7 October at 8pm ET/PT on The CW while DC's Legends Of Tomorrow will debut on The CW in early 2016.